Avalanche photodiodes (APDs) are widely utilized for fiber-optic communications due to the higher sensitivity benefitting from carrier multiplication mechanism. Traditional III-V APD receivers offer more than 6 dB sensitivity improvement up to 10 Gb/s data rate when compared to PIN photodiodes (PD). However, InP-based APDs show limited gain-bandwidth product and high multiplication noise due to large k-factor, i.e., ratio of impact ionization coefficients. Silicon-based APDs have been demonstrated to be the best device for high-speed communication applications. A common silicon-based APD utilizes a separated absorption, charge and multiplication (SACM) structure. A strained germanium (Ge) layer is adopted as the absorption layer, due to the high absorption efficiency of Ge to communication wavelength covering 850 nm˜1600 nm. An intrinsic silicon (Si) layer is often used as the multiplication region, due to smaller k-factor, which has low multiplication noise and larger gain-bandwidth product. A p-type charge layer is inserted between the Ge absorption layer and Si multiplication layer to provide high electric field in the multiplication layer for electron-triggered impact ionization process. The typical electric field required for Si avalanche is as high as 350˜400 kV/cm. Holes are generated during the multiplication process and travel toward Ge absorption layer and p-contact. As the holes approach near the GeSi interface, where the electric field is near 300 kV/cm, the holes gain high energy from the electric field but not high enough to trigger an ionization event in Si layer, since the electric field required for avalanche in Si is about 350 kV/cm for holes to overcome the energy loss during scattering and trigger the avalanche. Thus the hot hole-carriers carrying high energy are injected into Ge layer of narrower bandgap, where impact avalanche events are more easily triggered compared with silicon material. The carrier avalanche is a non-local process, which means the carrier gains energy continuously from the high electric field (E-field) during drifting, and the carrier can trigger an impact ionization process when the energy is high enough at a location away from where it gained energy. As high-energy holes travel through the GeSi interface into Ge layer, impact ionization events will probably happen in Ge since Ge has lower ionization threshold energy than Si. If avalanche happens in the Ge layer, the device response speed will be decreased and k-factor of the APD will be larger due to higher k-factor of Ge material, thus multiplication noise is also increased.
In some designs of the inventors of the present disclosure, a guard-ring structure is adopted along the periphery of the silicon mesa, to prevent high E-field from penetrating into Ge sidewall, where high leakage current might be induced. The electric field within the silicon guard-ring region is about 320 kV/cm under operation bias, only a bit lower than avalanche E-field, and this makes the silicon mesa sidewall a potential weak point.